Chronology of red dune aggradations of South India and its Palaeo-environmental significance
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Red sand dunes occur in the coastal plains of south east and west of Tamil Nadu, India between the coordinates of 8°00′ to 9°30′ N; 77°18′ to 79° 00′ E. OSL dating of these sands indicated aggradations between ∼16-9 ka and ∼9-3 ka in the west and east coasts respectively. Dating results from inland red dunes at the foothills of Western Ghats show a break in deposition at ∼6 ka and aggradation since ∼2 ka. The sand aggradations in the west coast occurred during the transition period when SW monsoon in the area was reestablishing. The dunes attained their stability by 9 ka. In the coastal region, the aggradations were controlled by sea level changes and a local recycling of earlier dunes (in the east coast). In the inland areas, the dune building was controlled by sand supply from fluvial sources.Keywords:
Foothills
Sand dune stabilization
Chronology
Aggradation
Lahar
Aggradation
Pumice
Hyperconcentrated flow
Debris flow
Conglomerate
Breccia
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In the Kumaun Himalaya, a portion of the Kosi River valley of ~90 km in length is chosen to study the fluvial morphology that provides first‐order information about the dynamic response of bedrock channels to tectonic impulse. The Kosi River flows across/along major tectonic boundaries such as the South Almora Thrust, Ramgarh Thrust, Main Boundary Thrust, and the Himalayan Frontal Thrust, and local transverse and longitudinal faults. Varied fluvial landforms correspond to different tectonic settings, lithologies, bedrock channels, hillslopes, large landslides, terraces, and fans. The longitudinal valleys are also the sites for thick aggradational landforms. Some portion of these valleys fall in the areas of active extensional tectonics and is characterized by one of the widest valley floor sections in the Lesser Himalaya. In contrast, the transverse valley sections are incised by deep‐cut v‐shaped valleys and the narrowest valley section. Swerving of the Kosi River is observed in the Ramgarh Thrust and Amel Fault zones and also in the Main Boundary Thrust zone. Recent tectonic activity is evident from the presence of the faulted Quaternary deposits, linear fault scarps, abandoned channels, incised meandering, and multiple levels of terraces/strath terraces. Field observations and the computed ratio of valley floor width to valley height (V f ) corroborate each other. Valleys developed parallel to the strike of faults and bedrocks have relatively broader valleys with higher V f values whereas in contrast, the valleys developed across the bedrock strike are narrow with smaller V f values. The results of computed stream length gradient (SL) and steepness (Ks) indices show considerable correlations between the obtained SL and Ks data and the field evidences; high values of SL and Ks are characterized by the presence of knick points observed at the prominent thrusts and faults.
Main Central Thrust
Bedrock
Landform
Thrust fault
Aggradation
River terraces
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Aggradation
Niger delta
Submarine canyon
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Aggradation
Denudation
Chronostratigraphy
Tectonic uplift
Chronology
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The Cloverly Formation of the western Powder River Basin represents nonmarine sedimentation during eastward migration of the Western Interior foreland basin coeval with orogenic activity in the Cordilleran fold-thrust belt. It provides an opportunity to evaluate aggradational responses to tectonic uplift. The Cloverly Formation overlies the Morrison Formation (Jurassic) with a sharp basal contact marked by a cross-bedded, pebble conglomerate consisting of black chert clasts; conglomerates are overlain by a thick, dominantly trough cross-bedded, medium to fine grained quartzarenite. Fining upward sequences are rare. Sedimentary structures suggest these sandstones to be deposited in fluvial channels with a highly sinuous northerly paleoflow. In outcrop, discontinuous sandstones represent distinct channel systems, with some channel sands being laterally adjacent to levee, overbank, and swamp facies of bioturbated, carbonaceous siltstones, mudstones, and thin coals. In subsurface, the Cloverly Formation thins and thickens over short horizontal distances, suggesting channel erosion into the upper Morrison Formation. Cloverly channel sands show width-depth ratios which are consistent with Holocene meandering systems. The Cloverly Formation was part of a rapidly aggrading alluvial plain with swamps and meandering fluvial systems. This aggradation limited development of fining upward sequences. Rapid aggradation was correlated also to either a large clastic influx frommore » the Sevier orogenic belt, or a rise in sea level causing aggradation within stream valleys, or both.« less
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Overbank
Crevasse
Alluvial fan
Conglomerate
Outcrop
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Abstract Alluvium in one reach of the Republican River valley, south-central Nebraska, dates to the Holocene. Radiocarbon dating of fills underlying the two terraces in the valley reveals that entrenchment between 4500 and 3700 yr B.P. formed the high terrace. Subsequently, alternating aggradation and stability occurred until sometime between 1800 and 1100 yr B.P. Following another period of alluviation, channel incision formed the low terrace. General synchrony between this chronology and other late Holocene fluvial chronologies of the central Great Plains suggests climatic control of fluvial activity. The episodes of incision are tentatively correlated with different climate regimes, illustrating the difficulty of attributing a given fluvial response to a particular direction of climatic change.
Aggradation
Chronology
Terrace (agriculture)
River terraces
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Sculpted rock bedforms (s-forms) on Whirlpool Sandstone are described from the bed of Twenty Mile Creek. The morphologies identified (undulating surfaces, rises, furrows, comma forms, spindles, potholes, and transverse troughs) are those described for sculpted bedrock forms (p-forms, s-forms) developed in subglacial environments. Additional morphologies identified are undercut downsteps and quarried surfaces in the lee of bedrock rises. Morphological differences between the subglacial and fluvial forms are attributed to the difference between confined conduit flow in the subglacial case and open-channel flow in the fluvial examples. In fluvial systems the descending bedrock surface presents rock differently to oncoming flows and favours certain s-forms (quarried lee faces and undulating surfaces). In a strong-flow fluvial environment s-forms develop by wear. Dominant sediment-transport modes are large clasts (up to metre dimensions) and suspended silt–clay with a small (< 7%) hard or heavy-mineral component. Hydraulic quarrying, which removes fracture-delimited blocks from the bed, interrupts the process. Although fluvial s-forms are similar to subglacial s-forms, large stable vortices may not exist in the fluvial context over the range of effective stages. Upper regime flow is common over bedrock reaches, and flow acceleration at small downsteps (knickpoints) magnifies the duration and spatial extent of wear effective velocities.
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<p>Alluvial fans and terraces are well developed and preserved in the foothills of the Eastern Cordillera of Colombia, being potential archives of tectonic uplift and climatic variations in a poorly studied tropical region. The main aim of the work is to investigate the evolution of fluvial terraces and alluvial fans in the upper Caqueta river basin. We used Landsat imagens and Copernicus digital elevation model (DEM; GLO-30) to map landforms and optically stimulated luminescence (OSL) dating in quartz grains to estimate burial ages. Our results show that most landforms have an alluvial origin, organized in wide fluvial fans with distinct lobes, at least two terrace levels (7 and 13 m above the river level), and connected incised floodplains along the landscape. The sedimentary facies of the fluvial fans and terraces are dominated by clast-supported massive gravels, with thin layers of coarse-grained sandstones. The modern floodplains are related with braided-like to sinuous channels from up to downstream; gravels dominate their sedimentary facies, but fine sand and mudstones are frequent. This sediment finning from terraces/fans to modern floodplains suggests a significant sediment-to-water discharge ratio change through the late Quaternary. New OSL ages will allow us to estimate the burial ages of the mapped landforms and give new insights into the timing and driving factors related to changes in the fluvial dynamic and landscape evolution of the tropical Andean piedmont. (FAPESP #2020/11047-1)</p>
Alluvial fan
Aggradation
Landform
Foothills
River terraces
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Foothills
Plio-Pleistocene
Aggradation
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